Method for baking photoresist applied on substrate

ABSTRACT

A photomask substrate, which is formed in a rectangular shape and on the surface of which photoresist is applied, is located above a lower heating plate so as to be separated from the lower heating plate by a proximity distance. The lower heating plate is provided with a heat unifying ring fixedly arranged thereon so as to surround the photomask substrate. The periphery of the heat unifying ring is provided with a taper section tapered at a predetermined angle to the periphery. The heat unifying ring has a container for containing the photomask substrate, at the central portion. An upper heating plate is arranged above the lower heating plate. The upper heating plate is provided with a baking chamber thereabove to cover the entire portion of the baking chamber section, thereby the baking chamber section can be shielded from the outside and free from the influence of the outside.

This is a division of application Ser. No. 08/654,885, now U.S. Pat. No.5,817,178 filed May. 29, 1996, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus and a method of performing abaking step in the photolithographic processes on a semiconductorsubstrate, a substrate of a liquid crystal display, and a substrate ofphotomask used for forming these substrates when the substrates areformed.

2. Description of the Related Art

The photomask used in semiconductor devices is obtained by a series ofsteps: forming of a thin film of metal such as Cr by use of thesputtering deposition; applying of a photoresist thereon; prebaking;writing of a pattern with an electron beam, writing apparatus;developing of the photoresist; postbaking; performing of aselective-etching of the thin metal film; and removing of thephotoresist.

In the conventional photolithography, the resolution or size uniformityof pattern on the surface of the substrate is not required so strictly.In addition, the characteristics of the photoresist used in thephotolithograpy do not much depend on the uneven distribution of heatapplied to the photoresist in the baking process.

The conventional baking apparatus for baking a photoresist will be morespecifically described below, in conjunction with the sectional viewshown in FIG. 1. As shown in the drawing, the conventional bakingapparatus has only one heating plate, i.e., heating plate 71. Asubstrate 72 made of material such as quartz and coated with photoresistis located above the heating plate 71 so as to be separated therefrom bya proximity gap of several microns. The photoresist is baked for apredetermined period of time. The proximity gap is provided by a sealingmember 73 between the substrate and the heating plate. As can beunderstood from this structure, the substrate 72 is placed in an openspace while the photoresist is being baked in the conventional bakingapparatus. The substrate is removed from the heating plate 71 so thatthe photoresist may be cooled at room temperature.

In recent years, as the semiconductor device is decreased in size,higher resolution and higher size uniformity are required of the patternon the substrate. Simultaneously, a chemical amplification electron-beamresist which can serve to provide a high-resolution pattern is put intopractical use. SAL 601, SAL 603, SAL 605 developed by Shipley Co., arethe most popular products of this type. Phenomenons such as crosslinkingreaction (in the negative type photoresist) and decomposing reaction (inthe positive type photoresist) are known to occur in this type ofphotoresist. As generally known, these reactions greatly influence thesize of the resist pattern during the heating step in the bakingprocess. In order to prevent the influence of these reactions in thechemical amplification electron-beam resist, the baking apparatuscapable of attaining the high size uniformity of the pattern isrequired.

However, in the conventional baking apparatus, the heating plate isdirectly influenced by the ambient air flows and the heat source. Alarge difference in temperature occurs in the surface of the heatingplate, i.e., between the central portion and the peripheral portion.When a 6 inch mask (a quartz substrate of a size of 152.4×152.4×6.4[mm]) is subjected to the baking step at 120° C., for example, themaximum temperature difference in the substrate is no less than 29.0° C.at the transition time the baking temperature is increasing (in thiscase, when the baking temperature is 63° C.). At the climax time thetemperature stably remains at 109° C., the maximum temperaturedifference is 7.9° C. As is clear from this, the maximum temperaturedifference in the substrate surface by use of the conventional bakingapparatus is considerably large both at the climax and during thetemperature transition.

A chemical amplification electron beam resist is generally baked atapproximately 100° C. to have its sensitivity amplified. At such atemperature, the sizes of the pattern will greatly change due to thetemperature difference in the surface of the substrate. The differencein the pattern size will greatly increase after the steps following thebaking step (i.e., the developing of the photoresist, selective etchingof the thin metal film, and removing of the photoresist) are performed.The resultant substrate submitted to such a process does not satisfy themanufacturing criteria, of course. This is thus critical problem to besolved urgently.

Some conventional baking process is performed without providing theaforementioned proximity gap between the heating plate and thesubstrate. In this case, the maximum temperature difference increasesmore.

Further, since the heating plate of the conventional baking apparatus isexposed to the outside, as shown in the drawing, and dust may stick tothe photoresist during the baking process. This structure of theconventional baking apparatus is thus also one of the causes ofdegradation of the photomask.

As described above, with the conventional baking apparatus, the maximumtemperature difference in the substrate surface increases during thebaking step, and the pattern size of the photomask (final product)greatly differs from the designed one.

SUMMARY OF THE INVENTION

The present invention thus designs to provide the apparatus for bakingphotoresist applied on a surface of a substrate, comprising a firstheating plate for heating the substrate, the substrate is located on thefirst heating plate so as to directly contact the first heating plate orto be separated therefrom by a proximity gap of several microns; asecond heating plate located above the substrate surface for heating thesubstrate; a heat unifying plate arranged on the first heating plate soas to surround the substrate; and a container containing the first andsecond heating plates and the heat unifying plate.

The present invention further designs to provide the apparatus forbaking photoresist applied on a surface of a substrate, comprising afirst heating plate for heating the substrate, the substrate is locatedon the first heating plate so as to directly contact the first heatingplate or to be separated therefrom by a proximity gap of severalmicrons; a second heating plate located above a substrate surface onwhich the photoresist is applied for heating the substrate; a heatunifying plate arranged on the first heating plate so as to surround thesubstrate; moving means for moving the second heating plate in adirection in which the second heating plate approaches the first heatingplate when the substrate is subjected to the baking step; and acontainer containing the first and second heating plates and the heatunifying plate.

The present invention still further provides the apparatus for bakingphotoresist applied on a surface of a substrate, comprising a firstheating plate for heating the substrate, the substrate is located on thefirst heating plate so as to directly contact the first heating plate orto be separated therefrom by a proximity gap of several microns; asecond heating plate for heating the substrate, which is located abovethe substrate surface on which the photoresist is applied, and arrangedsuch that the lower surface of the second heating plate approximates tothe peripheral portion of the substrate; a heat unifying plate arrangedon the first heating plate so as to surround the substrate; a containercontaining the first and second heating plates and the heat unifyingplate; and a cooling plate for cooling the substrate.

The present invention also provides the method of baking photoresistapplied on a surface of a substrate, comprising the steps of unifyingthe temperature of the substrate, by cooling the substrate before thesubstrate is subjected to the following baking step; baking thephotoresist applied on the substrate in a container, the substrate isarranged on the first heating plate, and the periphery of the substrateis surrounded by a heat unifying plate on the first heating plate, and asecond heating plate is arranged above the surface of the substrate; andcooling on a cooling plate the substrate subjected to the baking step bydischarging cooling gas from the above of the substrate.

The present invention additionally provides the method of bakingphotoresist applied on a surface of a substrate, comprising the steps ofbaking in a container the photoresist applied on the substrate, thesubstrate is arranged on the first heating plate, the periphery of thesubstrate is surrounded by a heat unifying plate on the first heatingplate, and a second heating plate is arranged above the surface ofsubstrate; and moving the second heating plate in a direction away fromthe first heating plate after the substrate is subjected to the bakingstep.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention and, together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a sectional view of the conventional baking apparatus;

FIG. 2 is a top view of the entire structure of the baking apparatus ofthe present invention used for baking a photomask substrate;

FIG. 3 is a sectional view more specifically illustrating the structureof the baking apparatus according to the first embodiment of the presentinvention;

FIG. 4 is a sectional view of a lower heating plate in the bakingchamber section shown in FIG. 3;

FIG. 5 is a top view of a heat unifying ring in the baking chambersection shown in FIG. 3;

FIG. 6 is a top view of an upper heating plate in the baking chambersection shown in FIG. 3;

FIG. 7A is the photolithography process using the chemical amplificationresist;

FIG. 7B is the photolithography process using the non-chemicalamplification resist;

FIG. 8 is a graph showing the average temperatures and the maximumtemperature differences in the conventional apparatus and the apparatusaccording to the present invention at the transition time when thebaking temperature is increasing.

FIG. 9 is a side view specifically illustrating the structure of acooling section of the baking apparatus shown in FIG. 2;

FIG. 10A is a sectional view of the first modification of the upperheating plate;

FIG. 10B is a sectional view of the second modification of the upperheating plate;

FIG. 10C is a sectional view of the third modification of the upperheating plate;

FIG. 10D is a sectional view of the fourth modification of the upperheating plate;

FIG. 11 is a sectional view of the modification of the baking chambersection of the baking apparatus shown in FIG. 2;

FIG. 12 is a sectional view of the modification of the lower heatingplate;

FIG. 13 is a sectional view specifically illustrating the structure ofthe baking chamber section according to the second embodiment of thepresent invention;

FIG. 14A is a graph of the descending speed of the heating plate in thebaking chamber section shown in FIG. 13;

FIG. 14B is a graph of the ascending speed of the heating plate in thebaking chamber section shown in FIG. 13;

FIG. 15 is a sectional view specifically illustrating the structure ofthe baking chamber section according to the third embodiment;

FIG. 16 is a plain view of the heat unifying ring according to amodification of the present invention; and

FIG. 17 is a sectional view of the heat unifying ring shown in FIG. 16taken along a line IXVII--IXVII.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described below in conjunction with thedrawings.

As shown in FIG. 2, the baking apparatus according to the presentinvention comprises baking chamber section 11, cooling section 12,carrying section 13, substrate carrier 14 and control panel 15.

In this structure, when the command is sent from the control panel 15, aphotomask substrates 16 contained in the substrate carrier 14 are pickedout by the carrying section 13 one by one, and carried to the coolingsection 12 or the baking chamber section 11 so as to be cooled orsubjected to the baking step. The photomask substrate 16 which have beensubjected to all steps in the baking process is carried back to thesubstrate carrier 14 by the carrying section 13 and contained therein.

In this baking apparatus, these steps of the baking process (i.e.,picking out from the substrate carrier 14, cooling, baking, cooling andcontaining the photomask substrate 16 in the substrate carrier 14) areserially and automatically performed in accordance with the inputprogram when the control command is sent from the control panel 15.

The baking chamber section 11 of FIG. 2 has a structure shown in FIG. 3,for performing the heating treatment of the substrate.

In FIG. 3, reference numeral 21 denotes a lower heating plate (a firstheating plate). A rectangular photomask substrate 16 on the surface ofwhich is applied with photoresist is located above the lower heatingplate 21 so as to be separated from the lower heating plate 21 by aproximity gap of several microns. The lower heating plate 21 is providedwith a groove 22 such that a substrate-carrying chuck (not shown) of thecarrying section 13 are put therein.

As shown in FIG. 4, the lower heating plate 21 including a heating plate41 and a heater 42 as heating means. In the central portion of theheating plate 41, a thermocouple 43 is embedded to adjust thetemperature. The heater 42 is controlled on the basis of the output ofthe temperature controller 44.

FIG. 3 shows a heat unifying ring (heat unifying plate) 23 fixed on thelower heating plate 21 so as to surround the periphery of the photomasksubstrate 16. The heat unifying ring 23 is formed to have substantiallythe same thickness as that of the photomask substrate 16. The peripheryof the heat unifying ring 23 is provided with a taper section 24 taperedto the periphery at a predetermined angle. The heat unifying ring 23 isfurther provided with a container 25, as shown in FIG. 5 for containingthe photomask substrate 16. The container 25 has a shape correspondingto that of the photomask 16 and a size slightly larger than thephotomask substrate 16.

FIG. 3 also shows an upper heating plate (a second heating plate) 26arranged above the lower heating plate 21. Similarly to the lowerheating plate 21, the upper heating plate 26 is also provided with aheating plate and a heater as a heat source. In addition thereto, athermocouple and a temperature controller are also provided to the upperheating plate 26, for controlling temperature. The upper heating plate26 is formed to have the smallest thickness at the central portion andto increase the thickness as the distance from the central portionincreases. The thickness of the upper heating plate 26 is set in such amanner in order to set the amount of the heat applied to the photomasksubstrate 16 located on the lower heating plate 21 to be smallest at thecentral portion and to gradually increase as the distance from thecentral portion increases.

Above the upper heating plate 26, surface of a baking chamber 27 isprovided to cover the entire portion of the baking chamber section 11,thereby the baking chamber section 11 can be shielded from the outsideand free from the influence of the outside.

The upper heating plate 26 is further provided with gas inlet holes 28at the portion located immediately above the taper section 24 on thelower heating plate 21. The gas inlet holes 28 are, as shown in FIG. 6,formed on the substantially entire periphery of the upper heating plate26 such that inert gas such as nitrogen gas discharged from a gassupplying means 29 shown in FIG. 3 flows through the gas inlet holes 28at a predetermined rate. By introducing the nitrogen gas flow into thechamber, the screen of the nitrogen gas is formed between the upperheating plate 26 and lower heating plate 21. The nitrogen gas flows ontothe taper section 24 formed on the heat unifying ring 23 and diffuses tothe outside of the portion in which the photomask substrate 16 islocated. A part of the nitrogen gas flows into the portion in which thephotomask substrate 16 is located. The photomask substrate 16 is chargedwhen it is exposed to the stream of the nitrogen gas. The temperature ofthe nitrogen gas is set at the temperature by which the uniformity ofthe temperature in the substrate is not adversely affected, or higherthan the temperature by several degrees. By introducing the nitrogen gasthe temperature and the flow rate of which are suitably set in such amanner, the uniformity of the temperature in the substrate can beimproved.

Further, as shown in FIGS. 3 and 6, an opening section 30 is provided atthe central portion of the upper heating plate 26, and an opening 31 isprovided to a portion of the chamber 27 just above the opening section30. The nitrogen gas is discharged from discharging means through theopening 31 at a predetermined flow rate.

Next, the photolithography process using the chemical amplificationelectron-beam resist will be described below.

The photolithography process using the chemical amplificationelectron-beam resist is generally performed in order shown in FIG. 7A.At first, the substrate on the surface of which a chrome film is formedis applied with photoresist, and then cooled to a room temperature. Thecooled substrate is subjected to the prebaking step and cooled to a roomtemperature again, and a pattern is written by means of EB (ElectronBeam) writing with an electron beam writing apparatus. Thereafter, thesubstrate is cooled to a room temperature again to perform the PEB (PostExposure Baking). After the substrate subjected to the baking step iscooled to room temperature again, the photoresist applied on thesubstrate is developed, and the selective etching of the chrome film andthe removal of the photoresist are followed thereto.

While, the photolithography process using the non-chemical amplificationelectron-beam resist is performed in order shown in FIG. 7B. The samesteps as in the photolithography using the chemical amplificationelectron-beam resist is also performed in the photolithography processusing the non-chemical amplification electron-beam resist till the EBstep. In the process using the non-chemical amplification electron-beamresist, the resist is developed after the EB step, and the cooling (to aroom temperature), postbaking, and cooling (to a room temperature) areperformed, and followed by the selective etching of the chrome film andthe removal of the photoresist, as in the process using the chemicalamplification electron-beam resist.

When the chemical amplification electron-beam resist is used, the bakingapparatus is used in the first baking process comprising the prebakingand cooling and the second baking process comprising the cooling, PEB,and cooling. The baking processes in the process using the chemicalamplification electron-beam resist will be more specifically describedin the following paragraphs. In the baking processes, a photomasksubstrate such as a 6 inch mask (a quartz substrate of a size of152.4×152.4×6.4 [mm]) is subjected to the baking processes, and thesupply flow rate of nitrogen gas supplied from the gas inlet holes 28 ofthe heating plate 26 can be set within a range of 0.5-5.0 l/min, and thedischarge flow rate of gas discharged from the opening 31 of the bakingchamber 27 can be set within a range of 0.1-3.0 l/min. The coolingsection 12 is provided with a cooling plate the temperature of which isset in advance and kept at a room temperature, 23° C., for example.

As a pretreatment of the first baking process, a chemical amplificationelectron-beam resist is dropped on the photomask substrate 16 from aphotoresist applying apparatus, and the substrates is spun such that aphotoresist film having a predetermined thickness is formed thereon.Then, an operation command is sent from the control panel 15 to thesubstrate carrier 14 after a plurality of the substrates 16 are set inthe substrate carrier 14. The substrate carrier 14 receiving the commandform the control panel carries one of the substrates 16 to the coolingsection 12 and the cooling section 12 cools the substrate on the coolingplate by force for a predetermined period of time. Subsequently, in thecondition where the supply and discharge flow rates of the gas in thechamber are set at 0.5 l/min and the temperatures of the lower heatingplate 21 and the upper heating plate 26 are set at 100° C. and 90° C.respectively, the substrate 16 is located on the lower heating plate 21in the chamber 27 to be submitted to the prebaking step. Thetemperatures of the lower and upper heating plates can be adjustedwithin a range of 60-200° C. In the prebaking step, the substrate 16 issurrounded by the heat unifying ring 23 provided on the lower heatingplate 21 such that heat is applied to the side walls of the substrate 16from the heat unifying ring. By virtue of this structure, the maximumtemperature difference between the central portion and the peripheralportion of the substrate 16 can be remarkably decreased.

In addition, the upper heating plate 26 the temperature of which can beadjusted desirably is located above the substrate 16, and thetemperature distribution on the substrate can be set even by applying aproper amount of heat to the entire substrate from the above. Further,as shown in the drawing, the upper heating plate 26 is formed to havethe smallest thickness at the central portion so as to increase thethickness as the distance from the central portion increases so that thecentral portion of the substrate is applied with the smallest amount ofthe heat and the amount of heat applied to the substrate graduallyincreases as the distance from the central portion increases. Thetemperature of the peripheral portion remains lowest in the substrateheated only by the lower heating plate 21. By forming the upper heatingplate in such a shape, the peripheral portion can be heated higher thanthe central portion by the upper heating plate, thereby the temperaturedifference in the substrate can be minimized.

The prebaking step is generally performed to evaporate a part of thesolvent contained in the photoresist applied to the substrate andthereby to improve the sensitivity of the photoresist. In addition tothis effect, the prebaking step plays an important roll in theuniformity of the size in the resist mask as a final product. Byimproving the uniformity of the temperature in the substrate during theprebaking step, the uniformity of the size in the resist mask as a finalproduct can be also improved.

After the prebaking step performed for a predetermined period of time(10 minutes, for example), the substrate 16 subjected to the prebakingstep is immediately carried to the cooling section 12 to be cooled for apredetermined period of time by force on the cooling plate thetemperature of which is maintained at 23° C., a room temperature.

The cooling section 12 used for cooling the substrate 16 is formed asshown in FIG. 9, for example. The substrate 16 is carried by thecarrying section 13 on the distal ends of four pins 52a-52d (52c and 52dare not shown in the drawing) of the cooling plate 51. A nozzle 53 ofthe cooling section 12 is provided with buffer plate 54 and gas guidingplate 55 at the distal end. The buffer plate 54 is fixed to the nozzle53 by a component through which gas can flow, for example, with a screw56 a part of the wall of which is cut away. The four pins 52a-52d arefixed on an air cylinder 57.

In the cooling section 12 having such a structure, then the air cylinder57 is driven to move down the four pins 52a-52d on which the substrate16 is arranged such that the cooling plate 51 comes close to thesubstrate 16 till the distance therebetween is reduced, to approximately0.1 mm. Then, the nozzle 53 on which the buffer plate 54 and gas guidingplate 55 are fixed is moved to position above the substrate 16 by amechanism not shown.

The pure nitrogen gas is then introduced into the cooling section fromthe gas inlet 58 of the nozzle 53. The introduced nitrogen gas flowsonto the buffer plate 54 and diffuses to the outside of the buffer plate54. The gas guiding plate 55 and the substrate 16 guide the nitrogen gasto make the gas flow above the substrate even, and the substrate 16 iscooled by the gas flow. In this embodiment, the gas guiding plate 55 isformed larger than the substrate 16 in order to make the gas flow abovethe substrate 16 even, and the substrate 16 can be evenly cooled.

When the gas is introduced, if the distance between the gas guidingplate 55 and the substrate 16 is set larger than 20 mm, a turbulent flowmay be generated and the gas may not flow evenly toward the outerperiphery of the gas guiding plate, with the result that the substratemay be cooled unevenly. The distance between the gas guiding plate 55and the substrate 16 therefore needs to be set less than 20 mm.

The flow rate of the nitrogen gas is also an important factor for thetemperature uniformity in the substrate. If the flow rate of thenitrogen gas is small, the cooling of the substrate 16 begins from theperiphery, and the temperature difference between the peripheral portionand the central portion will increase, with the result that the largedifference in the sensitivity of the photoresist may occur in thesubstrate. While, if the flow rate of the nitrogen gas is too large, thecooling of the substrate 16 begins from the central portion, and thelarge difference in the sensitivity of the photoresist in the substratemay also occur. According to the experiment by the inventors, thesubstrate can be evenly cooled within a range of 2-5° C. of thetemperature difference when the flow rate of the nitrogen gas is setwithin a range of 5-30 l/min.

The second baking process including the cooling, the PEB and the coolingsteps indicated in FIG. 7A will be described below. As in the firstbaking process, the substrate 16 on which the EB writing has finished iscarried to the cooling section 12 at first to be cooled on the coolingplate which is set and maintained at 23° C., the room temperature, byforce for a predetermined period of time. Subsequently, the substrate 16is located on the lower heating plate 21 in the chamber 27 to besubmitted to the baking step in the condition where the supply anddischarge flow rate of the gas in the chamber is set at 0.5 l/min, andthe temperatures of the lower heating plate 21 and the upper heatingplate 26 are set at 109° C. and 113° C. respectively.

Also in the PEB step, the sidewall of the substrate 16 is applied withheat from the heat unifying ring 23 provided on the lower heating plate21 to surround the substrate 16, and thus the temperature differencebetween the central portion and the peripheral portion of the substrate16 can be remarkably decreased. In addition, the temperature of theupper heating plate 26 which is located further from the substrate 16 incomparing with the lower heating plate 21 is set higher than that of thelower heating plate 21. The entire substrate is heated by the upperheating plate located above the substrate, thereby the temperaturedistribution on the substrate can be set even.

In the above example, the maximum temperature difference between thelower and upper heating plates is set at 4.0° C. This maximumtemperature difference therebetween is desirably set in a range of0.2-5.0° C., though the temperature difference is influenced by thedistance between the lower and upper heating plates. The distancebetween the lower and upper heating plates is defined in a range of0.5-10.0 mm in the second baking process.

Further, the upper heating plate 26 is formed to have the smallestthickness at the central portion so as to increase the thickness as thedistance from the central portion increases. By forming the upperheating plate, the central portion of the substrate is applied thesmallest amount of the heat and the amount of heat applied to thesubstrate gradually increases as the distance from the central portionincreases. The temperature of the peripheral portion remains lowest inthe substrate heated only by the lower heating plate 21. By forming theupper heating plate in such a shape, the peripheral portion can beheated higher then the central portion by the upper heating plate,thereby the temperature difference in the substrate can be minimized.

According to the experiment by the inventors, the maximum temperaturedifference in the substrate 16 is 3.0° C. before the temperature in thebaking chamber reaches the predetermined baking temperature and stillremains at 60° C. This value is considerably lower than the maximumtemperature difference 29.0° C. when the conventional apparatus is used.

FIG. 8 is a graph showing the average temperature and the maximumtemperature difference in the substrate when the substrate is heated inthe PEB process, which is prepared in order to compare the apparatus ofthe present invention and the conventional apparatus. As shown in thegraph, at the time when 60 seconds have passed from the beginning of thebaking, the maximum temperature difference in the substrate is 4.0° C.in the present invention, while that of the conventional apparatus is25.0° C. At the stabilizing time when 400 seconds have passed from thebeginning of the baking, the maximum temperature difference in thesubstrate when the conventional apparatus is used is 8.0° C. While, thepresent invention attain the maximum temperature difference of 0.2° C.

As shown in the above embodiment, the present invention considerablyimproves the temperature uniformity in the substrate in comparing withthe conventional apparatus.

The PEB step is an important step for the chemical amplificationelectron-beam resist since the difference in size in the resist masksignificantly depends thereon. During the PEB step the substrate iseasily influenced by the temperature difference of the heating plate andthe environment around the baking apparatus.

After the PEB step performed for 8 minutes, the substrate 16 isimmediately carried to the cooling section 12 to be cooled on thecooling plate which is set and maintained at 23° C., the roomtemperature, by force for a predetermined period of time. According tothe experiment by the inventors, the maximum temperature difference inthe substrate 16 is 4.5° C. during the cooling step. This is quite lowerthan that of the conventional apparatus.

The postbaking step indicated in FIG. 7A when the non-chemicalamplification electron-beam resist is used will be described below.After the photoresist is developed, the substrate is carried to thecooling section 12 to be cooled on the cooling plate which is set andmaintained at 23° C., the room temperature, by force for a predeterminedperiod of time. Subsequently, in the condition where the supply anddischarge flow rate of the gas in the chamber is set at 0.5 l/min andthe temperatures of the lower heating plate 21 and the upper heatingplate 26 are set both at 90° C., the substrate 16 is located on thelower heating plate 21 in the chamber 27 to be submitted to thepostbaking step for a predetermined period of time, 10 minutes, forexample. The postbaking step is intended to evaporate the developingsolution by heating the resist pattern which is expanded by thedeveloping solution, and thereby to shrink the pattern. The evenness ofthe size in the resist pattern as a final product can be improved alsoin this step. After the postbaking step has finished, the substrate isimmediately carried to the cooling section 12 to be cooled on thecooling plate which is set and maintained at 23° C., the roomtemperature, by force for a predetermined period of time.

The following is the description of the various modifications of thesubstrate baking apparatus described in the above embodiment. In theabove embodiment, the upper heating plate 26 has the smallest thicknessat the central portion, as shown in FIG. 3, so as to increase thethickness as the distance from the central portion increases so that thecentral portion of the substrate is applied the smallest amount of theheat and the amount of heat applied to the substrate gradually increasesas the distance from the central portion increases. The upper heatingplate 26, however, can be formed in any shape or structure, if theheating plate has the same function.

FIG. 10A shows the upper heating plate 26 having the smallest thicknessat the central portion such that the thickness of the heating platestepwisely increases as the distance from the central portion increases.

FIG. 10B shows the upper heating plate 26 having the same thickness atany portion. The upper heating plate 26 of FIG. 10B is bent at thecentral portion such that the central portion is the highest in theheating plate and furthest from the lower heating plate. The distance tothe lower heating plate gradually increase as the distance from thecentral portion increases.

FIG. 10C shows the upper heating plate 26 having a hole 61 in thecentral portion.

FIG. 10D shows the upper heating plate 26 formed so that the centralportion of the substrate is applied with the smallest amount of the heatand the amount of heat applied to the substrate gradually increases asthe distance from the central portion increases, by adjusting the heatsupplied from the heater 42 not shown in this drawing.

According to the above-described embodiment, the upper heating plate 26is provided with the gas inlet holes 28 to introduce the nitrogen gasand form the screen of the nitrogen gas between the upper and lowerheating plates. The gas inlet holes 28 may be formed in the lowerheating plate 21 as shown in FIG. 11, to introduce the nitrogen gas bythe gas supplying means 29 therefrom and form the nitrogen gas screen.

In the above embodiment, the structure of the upper surface of the lowerheating plate 21 on which the photomask substrate 16 is located is notspecifically described. In the lower heating plate, the central portionof the surface of the heating plate 41 forming the lower heating plate21 may be provided with a round depression, as shown in FIG. 12. Byvirtue of this depression, the temperature of the central portion of thesubstrate located on the lower heating plate can be decreased when thesubstrate is subjected to the baking step. The temperature of thecentral portion of the substrate depends on the diameter and depth ofthe depression 62. When the 6 inch mask is subjected to the baking stepand the diameter of the depression is 50 mm and the depth of thedepression is 0.2 mm, the temperature of the central portion of thesubstrate is lower by 0.5° C. than the average temperature of thesurface of the substrate. As is clear from this, the temperaturedifference can be remarkably reduced by providing this depression on thecentral portion.

The following is the description of the second embodiment of the bakingapparatus of the present invention.

The baking chamber section 11 shown in FIG. 13 has a driving mechanismfor driving the upper heating plate 26 in upper and lower directions.More specifically, the baking chamber section 11 of the secondembodiment, unlike the baking chamber shown in FIG. 3, provided with thedriving mechanism comprising arm 63 fixing the upper heating plate 26,ball screw 64 and motor 65, and drives the upper heating plate 26 inupper and lower direction by moving up and down an arm 63 fixing theupper heating plate 26.

In the baking chamber section 11 of the second embodiment, the upperheating plate 26 is moved up before the photomask substrate 16 islocated on the lower heating plate 21, in order to easily locate thephotomask substrate 16 on the lower heating plate 21, and when thesubstrate is subjected to the baking step, the upper heating plate 26 ismoved down to approach the lower heating plate 21.

It is necessary for the improvement of the temperature uniformity in thesurface of the substrate to increase the baking chamber at apredetermined temperature as soon as possible after the photomasksubstrate 16 is located on the lower heating plate 21. However, if themoving velocity of the upper heating plate 26 increases too much, dustmay be raised and sucked up by the upper heating plate 26. The suckeddust may stick to the surface of the photomask substrate 16 and may besubjected to the baking step on the substrate.

In order to prevent the stick of dust to the substrate during themoving-down of the upper heating plate 26, the rotating speed of thedriving motor 65 by which the moving velocity of the upper heating plateis defined is controlled such that the moving velocity V of the upperheating plate 26 is gradually decreased after the distance T between theupper and lower heating plates is decreased less than a predetermineddistance T₀ (5 mm, for example), as shown in FIG. 14A. By controllingthe driving motor 65 in such a manner, the stick of dust to thephotomask substrate can be prevented.

After the substrate is subjected to the baking step, the upper heatingplate 26 is moved up to remove the photomask substrate 16 located on thelower heating plate 21. In this time, the rotating speed of the drivingmotor 65 is controlled such that the moving velocity V of the upperheating plate 26 is gradually increased till the distance T between theupper and lower heating plates reaches the predetermined value T₀ (5 mm,for example), as shown in FIG. 14B. After the distance T increases morethan the predetermined distance T₀, the moving velocity V of the upperheating plate 26 is maintained at a constant high level at which highbaking throughput can be attained.

The photomask substrate 16 is raised away from the upper heating plate26 after the movement of the upper heating plate 26, as mentioned above.When the photomask substrate 16 is raised, air may flow into the regionunder the photomask substrate 16, charging the photomask substrate 16.In order to prevent the photomask substrate 16 from being charged, it israised in such a manner that the raising speed gradually increases atfirst and is then kept high.

As described above, when the upper heating plate 26 is located near thelower heating plate 21, the stick of dust to the photomask substrate 16can be prevented by decreasing the moving velocity V of the upperheating plate 26.

In addition, according to this embodiment, the distance between theupper heating plate 26 and lower heating plate 21 can be desirablyadjusted, and thus the distance can be set at an optimum level inaccordance with the thickness of the photomask substrate 16.

The third embodiment will be described below.

The environment in which the photomask substrate is located when thephotoresist subjected to the baking step by using the baking apparatusis quite dry. Therefore, when the baking step has finished and thephotomask substrate 16 is taken out, static electricity may be generatedon the surface of the photomask substrate 16, and dust may be easilystick to the substrate.

In order to prevent the stick of dust, the baking chamber section 11according to this embodiment is formed to have the baking chambersection 11 as shown in FIG. 13 an ionizer such as a soft X-ray ionizer(soft X-ray generator) 33, as shown in FIG. 15. In the baking chambersection 11 of this embodiment, like reference numerals are used todesignate like portions corresponding to those of the apparatus of FIG.13 throughout the drawings, for simplicity of illustration.

The soft X-ray ionizer 33 is arranged on the outer side wall of thebaking chamber 27 so as to emit the soft X-ray beam into the bakingchamber through a window 34 formed on the side wall of the bakingchamber 27. By providing the soft X-ray ionizer 33 with the bakingchamber in such a manner, the soft X-ray beam travels through a passagebetween the lower heating plate and upper heating plate in the lateraldirection, and the molecules in the atmosphere between the lower heatingplate and upper heating plate (i.e., the atmosphere indicated by theoblique lines in the drawings) are ionized.

The soft X-ray ionizer 33 is controlled to operate when the baking hasfinished and the upper heating plate is moved up, for example. The softX-ray ionizer 33 generates ions and the ions are positioned on thesurface of the photomask substrate 16. The ions will neutralize thestatic electricity exist on the surface of the photomask substrate 16when the upper heating plate is moved, and dust may not be stick to thesubstrate. Since, as described above, the photomask substrate 16 may becharged when it is raised away from the upper heating plate 26 at theend of baking, the soft X-ray ionizer 33 can be actuated then. By sodoing, the photomask substrate 16 is prevented from being charged.

In the above-mentioned embodiment, the soft X-ray ionizer 33 is arrangedon the baking chamber provided with the driving mechanism for movingup/down the upper heating plate 26. The soft X-ray ionizer 33, however,may be arranged on the baking chamber as shown in FIGS. 3 and 11 onwhich is not provided with the driving mechanism. Further, any otherbeam may be used instead of the soft X-ray beam if the beam can ionizethe molecules in the atmosphere without disturbing the gas in thechamber. In these cases as well, the photomask substrate 16 can beprevented from being charged by actuating the ionizer when the photomasksubstrate 16 is raised away from the upper heating plate 26 at the endof baking.

In the described embodiments and the modifications, the photomasksubstrate is submitted to the baking process. It is further understoodby those skilled in the art that the present invention may be alsoapplied to not only a semiconductor substrate such as silicon substrateand sapphire substrate, but also a substrate of the liquid crystaldisplay.

In addition, though the substrate is repeatedly cooled before and afterthe baking step in the described embodiments, the cooling steps can beomitted as the case may be. Similarly, in the embodiments, the electronbeam writing apparatus is used to write the pattern in the photoresist,but the pattern writing using UV light or X-ray beam can be performedthroughout the baking process.

Further, not only the device having a structure shown in FIG. 5, butalso the device having a structure shown in FIGS. 16 and 17 can be usedas the heat unifying ring 23 fixed on the lower heating plate 21.

FIGS. 16 and 17 are plain view and sectional view of the heat unifyingring 23. The heat unifying ring 23 shown in FIGS. 16 and 17, similarlyto the device shown in FIG. 5, has a container 25 for containing thephotomask substrate 16 and a taper section 24 (not shown in FIG. 17)formed on the periphery thereof. In the heat unifying ring 23 shown inFIGS. 16 and 17, groove 35 for guiding fluid is formed along theperiphery of the container 25.

The gas set at a predetermined temperature is supplied through thegroove when the heat unifying ring 23 is used for the baking process,thereby the temperature uniformity on the photomask substrate 16 can beimproved. Further, when ionized gas is supplied, through the groove, thestatic electricity can be prevented from generating, with the resultthat the substrate can be protected from dust.

The groove similarly to the groove 35 can be also formed on the upperheating plate 26.

As described above, the present invention can provide an apparatus andmethod of baking photoresist applied on the surface of a substrate,which can improve the temperature uniformity in the surface of thesubstrate submitted to the baking process.

What is claimed is:
 1. A method of baking photoresist applied on asurface of a substrate, comprising the steps of:unifying a temperatureof said substrate by arranging said substrate on a cooling, plate for apredetermined period of time before said photoresist is baked; bakingsaid substrate in a container, wherein said substrate is located withrespect to a first heating plate by a distance ranging from directcontact with the first heating plate to within several microns of thefirst heating plate, a heat unifying plate being mounted on said firstheating plate in such a manner as to surround a periphery of saidsubstrate, wherein said heat unifving plate has substantially the samethickness as that of the substrate, and a second heating plate beingarranged above said surface of said substrate.
 2. A method of bakingphotoresist applied on a surface of a substrate, comprising the stepsof:unifying a temperature of said substrate by cooling said substratebefore said photoresist is baked; baking said substrate in a container,wherein said substrate is located with respect to a first heating plateby a distance ranging from direct contact with the first heating plateto within several microns of the first heating plate, a heat unifyingplate being mounted on said first heating plate in such a manner as tosurround a periphery of said substrate, wherein said heat unifying platehas substantially the same thickness as that of the substrate, and asecond heating plate being arranged above said surface of saidsubstrate; and cooling said substrate on a cooling plate by dischargingcooling gas from said above of said substrate.
 3. A method of bakingphotoresist applied on a surface of a substrate, comprising the stepsof:arranging said substrate, in advance of baking the photoresist, at adistance from a first heating plate ranging from direct contact with thefirst heating plate to within several microns of the first heatingplate, and mounting a heat unifying plate which has substantially thesame thickness as that of said substrate on said first heating plate insuch a manner as to surround a periphery of said substrate; arranging asecond heating plate above said first heating plate; baking saidsubstrate in a container in which said second heating plate is moved toapproach said first heating plate; and moving said second heating platein a direction away from said first heating plate after said photoresistis baked.
 4. A method according to claim 3, wherein when said secondheating plate is moved in a direction in which said second heating plateapproaches said first heating plate, and after the distance between saidsecond heating plate and said first heating plate is less than apredetermined value, said second heating plate is gradually deceleratedas said second heating plate approaches said first heating plate, andwhen said second heating plate is moved in a direction in which saidsecond heating plate leaves from said first heating plate, said secondheating plate is gradually accelerated till the distance between saidsecond heating plate and said first heating plate reaches apredetermined value.
 5. A method according to any one of claims 1-3,wherein said photoresist is baked such that a temperature of said secondheating plate is higher than that of said first heating plate.
 6. Amethod according to claim 5, wherein said photoresist is baked such thata temperature difference between said first and second heating plates isdefined within a range of 0.2-5.0°C.
 7. A method according to any one ofclaims 1-3, wherein said photoresist is a chemical amplificationelectron-beam resist.
 8. A method according to any one of claims 1-3,wherein said photoresist is baked such that an amount of heat applied tosaid substrate gradually increases as a distance from a central portionof said substrate increases and a distance from a peripheral portion ofsaid substrate decreases.
 9. A method according to any one of claims1-3, wherein said second heating plate is provided with gas inlet holesformed on said peripheral portion in order to supply gas at apredetermined flow rate from said gas inlet holes toward said firstheating plate, and said photoresist on said substrate surrounded by saidfirst and second heating plates and a gas screen formed by a gas flowsupplied from said gas inlet holes is baked.
 10. A method according toclaim 9, wherein said container is provided with a gas dischargingopening through which said gas is discharged at a predetermined flowrate.
 11. A method according to any one of claims 1-3, wherein theatmosphere at least above said first heating plate is ionized byirradiating a gap between said first and second heating plates when saidphotoresist is baked.
 12. A method according to claim 11, wherein a softX-ray is used for irradiating said gap.
 13. A method according to anyone of claims 1-3, wherein said heat unifying plate has a containersection for containing said substrate and a groove formed along aperiphery of said container section, and fluid is supplied in saidgroove.
 14. A substrate baking method according to claim 13, whereinsaid fluid is ionized gas.